A heated slab as thick as several hundreds of millimeters, which is produced by continuous casting, etc., is rolled to thickness of several to several tens of millimeters by a hot strip mill comprising a roughing mill and a finishing mill. The finishing mill usually comprises 5 to 7 four-roll stands arranged in tandem. In the case of a seven-stand finishing mill, first to third stands are called “upstream stands,” and fourth to seventh stands are called “downstream stands.”
A working roll used in such a hot strip mill comprises an outer layer coming into contact with a hot thin strip, and an inner layer integrally fused to an inner surface of the outer layer. Because the outer layer in contact with a hot thin strip is subjected to a large thermal and mechanical rolling load by hot rolling in a certain period, its surface inevitably suffers damages such as wearing, roughening, heat cracking, etc. After removing these damages from the outer layer by machining, the working roll is used again for rolling. The removal of damages from an outer layer of the roll is called “damage-removing cutting.” The working roll is discarded, after it is cut to remove damages from the initial diameter to the minimum diameter usable for rolling (discard diameter). A diameter in a range from the initial diameter to the discard diameter is called an effective rolling diameter. The outer layer in the effective rolling diameter range desirably has excellent wear resistance, failure resistance and surface roughening resistance to prevent a large surface damage such as heat cracking.
As working rolls used in downstream finishing stands in hot strip mills, which are required to have excellent wear resistance, failure resistance and surface roughening resistance, proposals have conventionally been made to provide composite rolls comprising outer layers made of highly alloyed grain cast iron having good failure resistance, to which hard carbide-forming elements such as Mo, V, etc. are add to improve wear resistance. For example, JP 2004-82209 A proposes a centrifugally cast, hot-rolling composite roll comprising an outer shell layer having a chemical component comprising by mass 3.0-4.0% of C, 0.8-2.5% of Si, 0.2-1.2% of Mn, 3.0-5.0% of Ni, 0.5-2.5% of Cr, 0.1-3.0% of Mo, and 1.0-5.0% of V, the balance being Fe and inevitable impurities; and a shaft portion made of usual cast iron or spherical graphite cast iron containing 2.5-4.0% of C, the thickness T of the outer shell layer and the radius R of the shaft portion meeting the relation of 0.03≦T/R≦0.5. This composite roll has good seizure resistance and wear resistance. However, the outer layer of the hot-rolling composite roll has been getting required to have higher wear resistance.
Hot-rolling composite rolls having outer layers of high-speed steel having high wear resistance are also proposed. For example, as an outer layer of a composite roll used in upstream finishing stands of hot rolling, JP 08-020837 A discloses a high-speed steel outer layer of a rolling roll having a small friction coefficient, the outer layer comprising by weight 1.50-3.50% of C, 1.50% or less of Si, 1.20% or less of Mn, 5.50-12.00% of Cr, 2.00-8.00% of Mo, 3.00-10.00% of V, 0.60-7.00% of Nb, more than 0.01% and 0.200% or less of B, and more than 0.08% and 0.300% or less of N, the balance being Fe and inevitable impurities, and meeting the formula (1) of V+1.8 Nb≦7.5 C−6.0, and the formula (2) of 0.20≦Nb/V≦0.80. Though the seizure resistance of the outer layer is improved by the addition of B, the outer layer is still insufficient in wear resistance, failure resistance and surface roughening resistance, which are required for the outer layers of hot-rolling composite rolls.
JP 2005-264322 A discloses a hot-rolling composite roll comprising an outer layer having excellent seizure resistance, and an inner layer integrally fused to the outer layer, the outer layer having a composition comprising by mass 1.8-3.5% of C, 0.2-2% of Si, 0.2-2% of Mn, 4-15% of Cr, 2-10% of Mo, 3-10% of V, 0.1-0.6% of P, and 0.05-0.5% of B, the balance being Fe and inevitable impurities, the outer layer optionally containing 3% or less of Nb, 5% or less of W, 5% or less of Ni, and 2% or less of Co. JP 2005-264322 A describes that 0.03% or less of S may be contained. However, this outer layer is still insufficient in wear resistance, failure resistance and surface roughening resistance.
JP 10-008212 A discloses a hot-rolling role having at least an outer shell layer made of high-carbon high-speed steel comprising by weight 1.5-3% of C, 0.5-5% of Cr, 0.5-8% of Mo, 1-8% of V, more than 1% to 8% of W, 0.1-5% of Nb, and 0.01-1% of B, and containing 5-20% by area of MC carbide having particle sizes of 15 μm or less and a major diameter/minor diameter ratio of 2 or less in the structure. It describes that S is regarded as an inevitable impurity, which may be contained in an amount of 0.08% or less. However, the outer shell layer of the roll of JP 10-008212 A does not have sufficient wear resistance, failure resistance and surface roughening resistance.
JP 61-26758 A discloses a composite roll outer layer having excellent seizure resistance, which has a chemical composition comprising by weight 1.0-2.0% of C, 0.2-2.0% of Si, 0.5-1.5% of Mn, 3.0% or less of Ni, 2-5% of Cr, 3-10% of Mo, 4.0% or less of V, and 0.1-0.6% of S, the balance being substantially Fe. However, because this composite roll outer layer does not contain B at all, it still does not have sufficient wear resistance, failure resistance and surface roughening resistance.